Cement bonded articles are coated on their surface in order to avoid efflorescence and to attain an aesthetic appearance. In the course of time the coating is exposed to weathering through atmospheric conditions. While with strong sun exposure in the summer the temperature on the surface may increase up to about 80° C., with frost the temperature can decrease to −30° C. in winter. The attack by cycles of freezing and thawing, as well as by acid rain, is particularly critical.
Various coating compositions are known in the art as useful for protecting cement bonded articles from weathering. Among these, silica-based coating compositions may in particular be cited.
U.S. Pat. No. 2,956,958 discloses a silica dispersion useful for coating a substrate such as glass. This dispersion contains amorphous silica particles in the proportion by weight of (a) one part of particles having a diameter larger than 50 nm and an average diameter D from 50 to 150 nm; (b) from 0 to 0.07 part of particles from 0.25 D to 0.4 D nm in diameter; and (c) from 0.04 to (0.4-x) part of particles 4 to 0.25 D nm in diameter where x is the parts by weight of portion (b). The dispersion does not contain any polymer.
U.S. Pat. No. 3,895,956 discloses a water permeability reducing inorganic coating slurry composition useful for coating construction surfaces. The composition contains 5-700 parts by weight of an inorganic filler; 0.1-30 parts by weight of a water swellable magnesium silicate and/or bentonite; 0.1-50 parts by weight of a water-repellent (alkali alkyl siliconate, inorganic powder coated with alkali alkyl siliconate or fatty acid); 0.01-10 parts by weight of a viscosity improver (monovalent alkali metal, ammonium or amine salt of a polyacrylic acid having a degree of polymerization of at least 2000, polyethyleneoxide of 500,000 molecular weight, or water soluble cellulose derivative); and 100 parts by weight of a silica sol containing a silica component ranging from 3-50 wt % and at least one alkali component, wherein the mole ratio of SiO2 to M2O is greater than 4 (M2O being an alkali metal oxide, ammonium hemihydrate, an amine hemihydrate, guanidine hemihydrate or hydrazine hemihydrate).
U.S. Pat. No. 4,002,590 discloses a coating composition useful for coating construction surfaces. The composition comprises a coating liquid containing water, aqueous silica sol, an aqueous dispersion of butyl acrylate-methyl acrylate-styrene copolymer and an aggregate, wherein said coating liquid contains from 5 to 40 wt % as SiO2 of colloidal silica, 3 to 40 wt % of said copolymer, a SiO2/M2O mole ratio, wherein M is an alkali metal atom selected from the group consisting of Na, K, and Li, of from 7 to 2000 and has a viscosity lower than 100 c.p. at 20° C. when kept in a sealed state at 50° C. for 10 days.
U.S. Pat. No. 4,117,197 discloses a process for producing coated building components. The process comprises mixing an inorganic binder, water and aggregate to produce a workable mass, molding said mass into a preformed component, applying to the preformed component an aqueous paste containing a metal oxide and at least one of water-glass and a phosphate to form a coating and allowing the coating to harden. More precisely, about 190 to 400 g of the paste is applied per square meter of surface to be coated prior to hardening of such surface, the application being effected in the presence of at least about 0.5% by weight of the inorganic binder or, in the case of lime-sand bricks, by weight of the binder plus aggregate, of a water soluble inorganic salt which converts the aqueous paste into a gel-like non-flowing form, the inorganic salt being included in the foundation or protective layer. Preferred aqueous pastes comprise 42 to 63 mole % SiO2, 11 to 27 mole % alkali oxide and 19 to 42 mole % metal oxide. The metal oxide may be ZnO, MgO, PbO, CaO, B2O3 and/or Al2O3.
U.S. Pat. No. 4,153,591 discloses a coating composition useful for coating construction surfaces. The composition is prepared by adding an aggregate to a coating liquid containing 5 to 40 wt % as SiO2 of a colloidal silica and 3 to 40 wt % of dissolved and/or finely dispersed organic polymer, having an SiO2/M2O mole ratio (in which M is a monovalent alkali metal atom) of at least 7, and having a viscosity lower than 100 c.p. at 20° C. when kept in the sealed state at 50° C. for 10 days. The organic polymer may in particular be chosen from the group consisting of polyvinyl acetate, poly(methyl acrylate), poly(butyl acrylate), styrene-butyl acrylate copolymers, vinyl acetate-maleic acid copolymers, urea-formalin resins, melamine-formalin resins, natural rubber, styrene-butadiene copolymer rubbers, acrylonitrile-butadiene, copolymer rubbers, epoxy resins and polyurethane resins.
EP 0247910 discloses a coating composition for coating almost any type of material. The composition contains (a) about 100 parts by weight (based on solids) of a binder component prepared by thermally dissolving about 10 to about 40 parts by weight of fine particles of silica in about 100 parts by weight (based on solids) of potassium silicate and (b) about 15 to about 100 parts by weight of pearl luster pigment. The composition is inorganic and does not comprise any polymer.
U.S. Pat. No. 5,330,794 discloses a silica-based coating useful for coating glass, metal, ceramic and plastic. The coating comprises a binder and a filler. The binder is a mixture of silica-based polymers, colloidal silica, and metal oxide particles in an aqueous suspension. It includes four main ingredients: an alkali metal silicate; silica gel; a crosslinking agent such as methyltrimethoxysilane or tetraethoxysilane; and water. Typical fillers include clay, wollastonite, nepheline syenite, talc, flint, ceramic fibers, metallic fibers, ceramic microspheres, glass microspheres, cerium dioxide and dichromium trioxide. The specified ratio of SiO2 to alkali metal oxide is between 6.6:1 and 8.5:1.
U.S. Pat. No. 5,584,921 discloses coatings for building materials, in the form of an aqueous dispersion comprising: (a) 10 to 60 wt % of at least one polysiloxane; (b) 5 to 65 wt % of at least one colloidal silica in the form of silica sol; (c) 5 to 80 wt % of at least one pigment or unreactive filler; and (d) 0 to 30 wt % of at least one further varnish or paint additive, the sum of components (a), (b), (c) and (d) being 100 wt % on a water and solvent-free basis. As an example of polysiloxane is described an aqueous emulsion of a hydroxyl-functional methyl silicone resin of the average composition T210D23M4 having an average molecular weight of approximately 12,000.
U.S. Pat. No. 5,824,147 discloses a silicate compound useful for coating building materials. The compound consists of silicon dioxide, alkali oxide selected from the group consisting of lithium oxide, potassium oxide, sodium oxide and mixtures thereof and one or more oxides selected from the group consisting of aluminum oxide, calcium oxide, titanium dioxide, magnesium oxide, zirconium dioxide and boric oxide, wherein the mol ratio of silicon dioxide to alkali oxide is 4-25:1, the mol ratio of aluminum oxide to silicon dioxide is 0-0.8:1, the mol ratio of calcium oxide to silicon dioxide is 0-0.45:1, the mol ratio of titanium dioxide to silicon dioxide is 0-0.45:1, the mol ratio of magnesium oxide to silicon dioxide is 0-0.45:1, the mol ratio of zirconium dioxide to silicon dioxide is 0-0.45:1 and the mol ratio of boric oxide to silicon dioxide is 0-0.45:1, and wherein said composition hardens at a temperature less than 100° C. This compound does not contain any polymer.
CA 2373132 discloses a silicate composition which comprises at least one amorphous binder matrix containing alkali oxide and silicon dioxide. The alkali oxide is lithium, sodium and/or potassium oxide. The amorphous binder comprises for each mole of alkali oxide more than 25 moles of silicon dioxide. The amorphous binder matrix comprises in addition for every 1000 g silicon dioxide in uniform distribution 10 g to 150 g of an integrated silicon-containing hydrophobing additive and the silicate composition comprises for every 1000 g silicon dioxide 400 to 7000 g of a filler substance whose particles have a thickness of less than 200 μm. The hydrophobing additive is preferably a polysiloxane, especially with alkoxy- and/or hydroxyl-functional groups.
However, the above coating compositions do not enable to achieve completely satisfactory results. In particular adhesion of the coating to the substrate is still insufficient for some applications. The resistance to weathering by freeze thaw attack due to water penetration as measured by contact angle is also inadequate to protect cement bonded articles for the long term.
There is thus a need for a new coating composition able to protect cement bonded surfaces and the like against weathering, said composition having better adhesive properties with superior self cleaning properties.